1. Field of the Invention
The present invention relates to an apparatus and a method for defect detection using a charged particle beam, and more particularly to a charged particle beam defect detection apparatus and a method of using such an apparatus in which a charged particle beam such as an electron beam or an ion beam is used for observing and detecting defects on the surface of an object such as a semiconductor substrate or a liquid crystal substrate.
2. Description of the Background Art
Semiconductor elements are formed by using planar techniques to generate a fine pattern on the surface of a semiconductor substrate. Demands for smaller semiconductor elements have lead to these patterns becoming finer and more highly integrated. Charged particle beam microscopes which utilize the charged particles from electron beams and the like are used for conducting observations and defect inspections of the surface condition of such semiconductor elements. Currently, the most widely known and widely used charged particle microscope is the scanning electron microscope (SEM). In recent years, imaging electron microscopes have been proposed as an alternative to scanning electron microscopes, and the development of charged particle beam imaging projection optical systems for this type of mapping electron microscopes is being actively pursued. As follows is a brief description of the construction of a charged particle beam imaging projection optical system.
A primary electron beam is emitted as an illuminating electron beam from an electron gun which functions as a charged particle source, and this beam passes through a primary optical system which functions as an illumination optical system, and enters an electromagnetic prism known as an E cross B (Exc3x97B). Passage through the Exc3x97B converts the cross-sectional shape of the primary electron beam to a linear shape, a rectangular shape, a circular shape or an oval shape, and the shaped beam then passes through a cathode lens which functions as an object optical system, and is illuminated onto the surface of a sample object. When the primary electron beam is irradiated onto the surface of an object, a reflected electron beam having a comparatively high energy is produced by reflection of the primary electron beam off the object surface, and furthermore, a secondary electron beam having a low energy is emitted from the object surface.
Of these two electron beams emitted from the object surface, the secondary electron beam is typically used for image generation. This secondary electron beam, which functions as an observation electron beam, passes through the cathode lens and enters the Exc3x97B. Following passage through the Exc3x97B, the secondary electron beam passes through a secondary optical system, which functions as an imaging optical system, and enters an electron beam detector. Observation and defect inspection of the object surface is then performed based on information obtained from injection of the secondary electron beam into the electron beam detector.
By the way, in devices such as the scanning electron microscope and the imaging electron microscope described above, where the observation and defect inspection of an object is carried out by the irradiation of charged particles such as an electron beam onto the object, because charged particles are irradiated onto the surface of the sample, the sample itself is charged up. Even if a charged particle beam with a uniform distribution relative to the sample surface is used, the amount of this charge-up will differ depending on the sample material. Therefore, in the case of a semiconductor element for example, the amount of charge-up in those areas where wiring is formed will differ from the amount of charge-up in those areas where an oxidation inhibiting film is formed, and so a charge-up distribution (a surface voltage distribution corresponding with the amount of accumulated charge on the object) generates.
Furthermore, the initial energy of a secondary electron beam generated in a section where charge-up has occurred will differ from the initial energy of a secondary electron beam generated in a section where absolutely no charge-up has occurred. Therefore, even if the focal position of the secondary optical system is adjusted so that the secondary electron beam from an area of no charge-up undergoes imaging onto the electron beam detector, the same focal position will not match the secondary electron beam emitted from an area where charge-up occurs. As a result, in order to ensure a more accurate observation of those areas where this charge-up phenomenon has occurred, the secondary optical system needs to be controlled and a correction made for this deviation in focal position. However, in order to correct this type of deviation in focal position, an operator must perform a manual correction for each sample, which is an extremely complex operation.
An object of the present invention is to provide a charged particle beam defect detection apparatus and a charged particle beam defect detection method, in which, even if charge-up occurs on the sample being observed, the focal position deviation resulting from this charge-up can be corrected without causing inconvenience to the operator, and a clear, in-focus observation and a highly accurate defect inspection can be performed.
In order to achieve the above object, a charged particle beam defect detection apparatus according to a first aspect of the present invention comprises an irradiation device which irradiates a beam from a charged particle beam source as a primary beam onto an object, an electron detection device which detects electrons emitted from the object as a result of the primary beam irradiation as a secondary beam and captures an image of the object, and a detection device which detects a surface voltage distribution for the object which corresponds with the amount of accumulated charge generated on the object upon irradiation with the primary beam.
According to this aspect of the invention, the surface voltage distribution for the object, corresponding with the amount of accumulated charge generated on the object upon irradiation with the primary beam, is detected with the detection device, and so information can be obtained which resolves the problems (such as focal position deviation and image distortion) due to accumulated charge on the object.
A charged particle beam defect detection apparatus according to a second aspect of the present invention comprises an irradiation device which irradiates a beam from a charged particle beam source as a primary beam onto an object, an electron detection device which detects electrons emitted from the object as a result of the primary beam irradiation as a secondary beam and captures an image of the object, a focus deviation detection device which detects in advance the degree of focus deviation of the secondary beam at the detection surface of the electron detection device, which corresponds with the amount of accumulated charge generated on the object upon irradiation with the primary beam, and a focus control device which controls the focal position of the secondary beam in accordance with the degree of focus deviation detected by the focus deviation detection device.
According to this aspect of the invention, the degree of focus deviation of the secondary beam at the detection surface corresponding with the amount of accumulated charge generated on the object upon irradiation with the primary beam, is detected in advance by the focus deviation detection device, and the focus control device then corrects the focal position of the secondary beam in accordance with this detected degree of focus deviation. Therefore, even in those cases where accumulated charge is generated on the object, the image of the object can be displayed in a focused state, and a clear and in-focus observation and a highly accurate defect inspection can be performed.
A charged particle beam defect detection apparatus according to a third aspect of the present invention is a charged particle beam defect detection apparatus according to the second aspect, in which a storage device is provided for storing the focus deviation values detected by the focus deviation detection device, and the aforementioned focus control device controls the focal position of the secondary beam based on the focus deviation values stored in the storage device.
According to this third aspect of the invention, the focus deviations values for the secondary electron beam resulting from accumulated charge on the object are stored in advance in the storage device, and during observation of the object, these deviation values are read from the storage device and used for controlling the focal position of the secondary beam. Therefore, in the case where, for example, the object is a semiconductor substrate with a plurality of shot areas with identical patterns set on the surface of the substrate, there is no necessity to detect the focus deviation of the secondary electron beam for each shot area, but rather the degree of focus deviation can be detected for just one of the shot areas. Furthermore, when observations are carried out for each of the shot areas, because the focal position can be controlled using the stored common focus deviation value, the throughput, namely the number of objects which can be observed within a unit of time, can be increased. Furthermore, in the case where a plurality of objects from an identical processing step, for example a plurality of substrates from a single lot, are subject to observation, then once again there is no necessity to detect the focus deviation for each object, and so the invention can also contribute to an increase in throughput in the case where observations should be performed for a plurality of objects.
A charged particle beam defect detection apparatus according to a fourth aspect of the present invention is a charged particle beam defect detection apparatus according to either one of the second aspect and the third aspect, in which a height detection device is provided for detecting the height of an object, and the aforementioned focus control device controls the focal position of the secondary beam based on both the aforementioned focus deviation values and the detection results from the height detection device.
According to this aspect of the invention, not only is the secondary electron beam focus deviation resulting from accumulated charge on the object corrected, but the height position of the object is also detected, and the thus detected height is also considered in controlling the focal position of the secondary beam. Therefore, even in those cases where, for example, the object is warped, the image of the object can be displayed in a focused state, and a clear and in-focus observation and a highly accurate defect inspection can be performed.
A charged particle beam defect detection apparatus according to a fifth aspect of the present invention is a charged particle beam defect detection apparatus according to the fourth aspect, in which the aforementioned storage device stores the object height values detected by the height detection device in correlation with the corresponding focus detection values, and the focus control device then controls the focal position of the secondary beam based on both the focus deviation values and the object height values correlated by the storage device.
According to this fifth aspect of the invention, the detected object height values and the detected focus deviation values are stored in a correlated manner, and when an observation is carried out, both these sets of stored data are used in controlling the focal position of the secondary beam. As a result, even if the object is warped, a clear, in-focus observation and a highly accurate defect inspection can be performed with a high level of throughput.
A charged particle beam defect detection apparatus according to a sixth aspect of the present invention is a charged particle beam defect detection apparatus according to any one of the second aspect through to the fifth aspect, wherein an imaging electron optical system is provided between the aforementioned electron detection device and the object, for imaging the secondary beam onto the detection surface of the electron detection device, and the aforementioned focus control device controls the focal position of the secondary beam by controlling the imaging electron optical system.
A charged particle beam defect detection apparatus according to a seventh aspect of the present invention is a charged particle beam defect detection apparatus according to any one of the second aspect through to the fifth aspect, in which a voltage application device is provided for applying a predetermined voltage to the object, and the aforementioned focus control device then controls the focal position of the secondary beam by controlling the voltage applied to the object via the voltage application device.
According to this aspect of the invention, the focal position of the secondary beam can be controlled without any complex control of the imaging electron optical system, simply by changing the voltage applied to the object, and so controlling the focal position of the secondary beam is simplified. As a result, not only can throughput be improved, but a clear, in-focus observation and a highly accurate defect inspection can be carried out with ease.
A charged particle beam defect detection apparatus according to an eighth aspect of the present invention is a charged particle beam defect detection apparatus according to the seventh aspect, in which the aforementioned focus control device controls the focal position of the secondary beam by controlling the voltage applied to the object based on the focus deviation values stored in the aforementioned storage device.
According to this eighth aspect of the invention, the focus deviations values for the secondary electron beam resulting from accumulated charge on the object are stored in advance in the storage device, and during observation of the object, these deviation values are read from the storage device and used for controlling the focal position of the secondary beam by controlling the voltage applied to the object. Therefore, in the same manner as the charged particle beam defect detection apparatus according to the third aspect, in the case where, for example, the object is a semiconductor substrate with a plurality of shot areas with identical patterns set on the surface of the substrate, there is no necessity to detect the focus deviation of the secondary electron beam for each shot area, but rather the degree of focus deviation can be detected for just one of the shot areas. Furthermore, when observations are carried out for each of the shot areas, because the focal position can be controlled using the stored common focus deviation value, the throughput, namely the number of objects which can be observed within a unit of time, can be increased. Furthermore, in the case where a plurality of objects from an identical processing step, for example a plurality of substrates from a single lot, are subject to observation, then once again there is no necessity to detect the focus deviation for each object, and so the invention can also contribute to an increase in throughput in the case where observations are to be performed for a plurality of objects.
A charged particle beam defect detection apparatus according to a ninth aspect of the present invention is a charged particle beam defect detection apparatus according to either one of the second and the third aspects, further comprising a height detection device which detects the height of the object, an imaging electron optical system provided between the aforementioned electron detection device and the object, for imaging the secondary beam onto the detection surface of the electron detection device, and a focused position calculation device which determines by simulation the relationship between the height of the object and the focused position of the imaging electron optical system relative to the detection surface of the aforementioned electron detection device in those cases where no accumulated charge exists on the object. The focus deviation detection device varies the focal position of the imaging electron optical system and saves the object height values detected by the height detection device together with the imaging results from the electron detection device, and then based on the difference between the focal position of the imaging electron optical system at the saved height value corresponding with the focused imaging result, and the focused position of the imaging electron optical system corresponding with the aforementioned height value as determined by the focused position calculation device, determines the degree of secondary beam focus deviation at the detection surface of the electron detection device which corresponds with the amount of accumulated charge.
A charged particle beam defect detection apparatus according to a tenth aspect of the present invention is a charged particle beam defect detection apparatus according to either one of the second and the first aspects, further comprising a voltage application device which applies a predetermined voltage to the object, a height detection device which detects the height of the object, an imaging electron optical system provided between the aforementioned electron detection device and the object, for imaging the secondary beam onto the detection surface of the electron detection device, and a focused position calculation device which determines by simulation the relationship between the height of the object and the focused position of the imaging electron optical system relative to the detection surface of the electron detection device in those cases where no accumulated charge exists on the object. The focus deviation detection device varies the voltage applied to the object via the voltage application device and saves the imaging results from the electron detection device when the focal position of the imaging electron optical system is matched with the focused position as determined by the focused position calculation device in accordance with the height values obtained by the height detection device, and then based on the amount of variation in the voltage applied to the body in the case where a focused imaging result is obtained, determines the degree of secondary beam focus deviation at the detection surface of the electron detection device which corresponds with the amount of accumulated charge.
According to these aspects of the invention, the height of the object is detected by the height detection device, and the height of the object is then taken into consideration in determining the degree of focus deviation resulting from accumulated charge on the object. Therefore, the construction of the charged particle beam defect detection apparatus can be simplified, and focus deviations resulting from variations in the height position of the object can be separated from focus deviations resulting from accumulated charge on the object even if the apparatus is not provided with a stage for varying the height position of the object. As a result, these aspects of the invention can be favorably applied to the correction of focus deviation resulting from accumulated charge even in those cases where the height of the object cannot be changed.
A charged particle beam defect detection method according to the present invention is a charged particle beam defect detection method for detecting defects in the surface of an object, by irradiating a beam from a charged particle beam source as a primary beam onto the object, detecting electrons emitted from the object as a secondary beam, and capturing an image of the object. The degree of focus deviation of the secondary beam which corresponds with the amount of accumulated charge generated on the object upon irradiation with the primary beam is detected in advance, and the focal position of the secondary electron beam is then controlled in accordance with this detected degree of focus deviation.
According to this charged particle beam defect detection method of the present invention, the same effects can be achieved as those described for the charged particle beam defect detection apparatus of the present invention.